The United States may rejoin Iter, the international consortium to build an experimental fusion reactor. It is just three years since the Americans walked away from the project, complaining about excessive costs and technical issues.

President Bush's science advisor and director of the Office of Science and Technology Policy, John Marburger III, says the US is now reviewing its position.

Iter, the International Thermonuclear Experimental Reactor, will be built in the next few years. A decision on its site, likely to be in Canada, Japan or France, is expected in 2003.

The reactor aims to produce energy in the same way as the Sun, by forcing together atoms at very high temperatures (about 100 million degrees Celsius).

High expectations

Fusion energy offers many advantages over "conventional" nuclear power, which works by fission, or the splitting of atoms.

John Marburger III says the US may go back into the Iter project

Fusion reactors could use seawater as a source of fuel. They would not emit greenhouse gases like fossil fuel power stations and neither would they create the highly radioactive waste found in current nuclear stations.

But exploiting fusion power has not been easy. The initial high expectations of researchers in the 1950s were quickly dampened when they realised how difficult it would be to solve some of the technical problems involved.

These included learning how to control the complex behaviour of an electrically charged gas, or plasma. The plasma, which contains the atomic nuclei to be forced together, is held in place inside the reactor by huge magnets that make the reactor look like a giant doughnut.

Final leap

It has taken several decades of research, at places like the Jet (Joint European Torus) project in Britain, to get a reactor to produce even a small amount of fusion energy.

Inside the Jet "doughnut"

Iter could be the final stage before the world sees the first commercial fusion power station.

In 1999, the US balked at Iter's $10bn price tag. Since it left the project, Iter scientists have revised the project and the expected cost now stands at about $4.5bn.

Those involved in the programme are urging the US to make its mind up soon, as the decision on where to build the experimental reactor must be made within a year. Privately they are hoping that the US joins without interfering with the choice between Canada, Japan or France.